1. Field of the Invention
This invention relates to a method and apparatus for electrowinning metals from a solution containing metals. This invention is particularly concerned with the production of particulate metal, eg in the form of powder, as distinct from plated metal.
This invention relates particularly but not exclusively to a method and apparatus for electrowinning copper in a powder form from a copper bearing solution, eg a low grade copper solution such as is often found at mines and mineral processing sites, and it will be convenient to hereinafter describe the invention with reference to this example application. However, it is to be clearly understood that the invention also applies to other metals, eg silver, nickel, cobalt and tin.
2. Description of the Related Art
The applicant has previously designed an electrowinning cell for electrowinning metals such as copper and tin from aqueous solutions. The cell is disclosed in the applicant""s international patent application number (WO 96/38602) entitled mineral recovery apparatus. The entire contents of this specification are explicitly incorporated into this document by cross-reference.
The above application discloses a cell having a tangential inlet at the bottom of the housing and a tangential outlet at the top of the housing. The orientation of the inlet which directs solution into the cell with a particular orientation in conjunction with the cylindrical housing induces a helical spiral flow through the cell. A rod-like anode extends axially the length of the housing coaxial with the cell and a split sleeve cylindrical cathode bears against the wall of the housing and circumferentially surrounds the anode spaced outwardly therefrom. In use, a potential difference is applied across the flow passage between the cathode and the anode to drive the electrowinning metal production process. The helical flow through the cell from the inlet to the outlet presents copper ions to the cathode continuously to plate out the copper economically even in low grade solutions.
This process progessively plates out a copper tube on the inside of the split sleeve. When the copper plate is about 6 to 8 cm thick (2.4 to 3.14 inches) it is harvested. This is accomplished by removing a top end cap from the cell and lifting the split sleeve out through the top of the cell. This is a labor intensive process and interferes with the otherwise continuous nature of the process.
As a commercial plant using the process contains banks of literally hundreds of cells, the harvesting of the cells in the manner described above is a labor-intensive process. A further disadvantage of the production of copper tubes in the manner described above is that the tubes require specific handling and transport procedures. It would therefore be advantageous if an easier method for harvesting the copper from the electrowinning cells could be devised.
In addition, the electrowinning cell described above may have less than optimum efficiency because of the large gap or distance between the cathode and anode. As a result, a relatively high voltage has to be applied across the cathode and anode and the applicable current density is relatively lower. As the amount of metal produced is directly proportional to the current density across the cathode and anode, it is desirable to have as high a current density per unit amount of electrical power input as possible.
According to a first aspect of this invention there is provided a cell for electrowinning a metal in powder form from solution, the cell including:
a housing having an inlet towards one end thereof and an outlet towards an opposed end;
an anode extending substantially axially through the housing;
a cathode surrounding the anode spaced outwardly away from the anode to define a flow passage between the cathode and anode, having a gap of 5 to 25 millimeters (0.20-0.98 inch); and
means for applying a potential difference between the anode and the cathode.
The cell therefore has a substantially narrower gap between the cathode and anode than either electrowinning plate cells or cylindrical cells for producing copper tubes. This assists in increasing the current density between the cathode and the anode, particularly for low conductivity solutions.
More preferably the gap is 5 to 20 millimeters (0.20 to 0.80 inches), even more preferably 10 to 15 millimeters (0.40 to 0.60 inches), most preferably 12 to 13 millimeters (0.47 to 0.51 inches).
Typically, both the anode and the cathode are substantially cylindrical. The cathode may be formed by the wall of the housing or by a sleeve positioned adjacent the wall of the housing. Preferably the cathode is formed by the wall of the housing which is metallic.
Typically one end of the cell has a relatively upper orientation and an opposed end of the cell has a relatively lower orientation in use, and the inlet is positioned at or adjacent the lower end and the outlet is positioned at or adjacent the upper end.
Thus, in use, process solution containing metal ions to be electrowon travels upwardly through the cell from the inlet to the outlet and metal is deposited on the cathode as a powder. Periodically, a flush solution is pumped in a reverse direction through the cell to remove deposited powder metal from the cell for harvesting. It is preferred that the process solution travels up through the cells so that gas generated by the electrowinning process can be vented through a vent associated with an upper region of the cell. It is particularly preferred that flush solution travels downwardly through the cell so that gravity assists with the flushing process. Typically, flushing would be assisted by other factors such as increased pressure of flush solution and passing air bubbles or other means over the cathode to assist in loosening the metal powder.
Preferably, the inlet directs solution into the cell in substantially an axial direction.
Preferably, the outlet is oriented such that flushing fluid which is passed through the cell in a reverse direction is directed axially into the cell through the outlet.
In a preferred form said inlet is defined in said one end of the cell and said outlet is defined in said opposed end of the cell.
The orientation of the inlet and gap of the flow passage facilitates process solution flowing through the flow passage with a turbulent flow. This is quite different from the tangential inlet in the prior art cell which induces a helically spiralling plug flow through the cell from inlet to outlet. Plug flow is fundamentally different from turbulent flow. Turbulent flow assists with the formation of powder metal as distinct from plate metal.
It is similarly advantageous that the flush solution which flows in a reverse direction through the outlet of the cell is directed axially into the cell to promote turbulent flow. This turbulent flow of flush solution assists in dislodging the metal powder from the cathode.
Preferably, the cell further includes means for guiding powder which is washed off the cathode during a flush cycle towards the inlet through which it is drained from the cell, eg a sloping internal surface of the housing.
This reduces the likelihood of metal powder collecting in dead spaces in the bottom of the cell and assists in fully draining metal powder from the cell.
Preferably, the cell further includes cleaning means for clearing metal plate obstructions from the flow passage between the anode and cathode of particulate metal and the cleaning means comprises a mechanical cleaner which is physically moved along the flow passage.
Naturally the process flow parameters are set so as to reduce the likelihood of solid metal, eg dendrites of metal, from depositing on the cathode. Applicant therefore believes that it is highly unlikely that metal plate obstructions such as dendrites will form in the flow passage. However, it is still necessary to provide a means for checking for and removing blockages of metal should they occur to provide a reliable piece of process equipment for use in a commercial plant.
Preferably, the ends of the anode are closed to direct fluid around the anode and through the annular flow passage. One end has a flow formation having a broadly conical configuration for directing flush solution passing through the outlet towards the flow passage. The closed ends ensure that solution flows around the anode and through the flow passage.
The cell may also include a support for supporting the anode in the form of a support member mounted to an end of the housing and projecting substantially axially into the housing. The support member mechanically supports the anode in the appropriate position vertically aligned with the cathode and also electrically connects the anode to the electrical circuit.
In a particularly preferred form the housing comprises a cylindrical body of stainless steel and end caps of non-conductive material on each end of the cylindrical body, each of the end caps defining a chamber positioned axially outwardly of the cathode and anode. One of the end chambers may form the sloping internal surface described above for guiding powder metal through the inlet.
This way the cylindrical body which forms the cathode is electrically isolated from the support member and electrical connection to the anode which passes through one of the end caps.
A particularly preferred form of the cell has a cathode with a diameter of 7xc2xd to 8xc2xd inches (190 to 216 mm), preferably about 8 inches (203 mm), and an anode with a diameter of 6xc2xd to 7xc2xd inches (165 to 190 mm), preferably about 7 inches (178 mm), with the gap between the anode and cathode being 0.5 to 1.5 inches, preferably about 1 inch (25.4 mm). Further, in the most preferred form, the housing is substantially vertically extending and the inlet is defined in the end of the lower end cap and the outlet is defined in the end of the upper end cap.
The cell may also include means for bubbling gas up through the flow passage. The bubbling means may comprise an apertured pipe positioned in the bottom of the chamber through which eg air is passed.
According to another aspect of this invention there is provided a bank of cells including:
a plurality of cells as defined above with respect to the first aspect of the invention, arranged in parallel;
an inlet main coupled directly to the inlet of each of the cells in the bank for directing process solution through the cells in parallel;
an outlet main coupled directly to the outlets of each of the cells for directing process solution away from the cells; and
means for interrupting a flow of process solution through the bank of cells when required and then passing a flush solution in a reverse direction through the outlet main, then through each of the cells in the bank, and then out through the inlet main.
In use, therefore, process solution is passed in parallel through each of the cells of the bank and flush solution in turn is periodically or intermittently passed in a reverse direction in parallel through the cells to flush the powder metal out of the cells.
Preferably, the flow reversal means includes a process solution inlet valve means for opening and shutting off the flow of process solution into the inlet main, and process solution outlet valve means for opening and shutting off the flow of process solution out of the outlet main in a downstream direction and also flush solution inlet valve means for opening and shutting off the flow of process solution into the outlet main, and flush solution outlet valve means for opening and shutting off the flow of flush solution out of the inlet main.
Thus, control of respectively process and flush solution flow through the bank of cells can be accomplished by an inlet and outlet main and single sets of valves associated with each of the process and flush solutions. This is a fairly simple reticulation and valve arrangement for a bank having a number of cells. It is far simpler than having a separate valve arrangement for each cell.
The bank may further include control means for controlling the valves eg to permit only flush solution or process solution to flow through the bank at one time. Many different control means may be used but a PLC controller is particularly useful.
The control of the valve means can be accomplished in a variety of ways including by manual control. The PLC controller is a proven piece of off-the-shelf equipment that can be used to reliably control the process.
Typically, the bank will also include means for venting gas generated by the electrowinning process from the cells in the bank. Typically, the venting means comprises a vent operatively coupled to the outlet main.
The vent is important for removing gas generated by the electrowinning process in a commercial plant. By having the outlet main operatively coupled to the outlets of each of the cells a single vent can be used to vent all the cells in a bank. It is considerably simpler and cheaper than having a vent for each cell.
Preferably, the inlet main is adjacent a lower end of each of the cells and the outlet main is adjacent an upper end of the cells. Naturally, the inlet and outlet main will be positioned so as to minimise the length of piping required.
According to yet another aspect of this invention, there is provided a method of operating an electrowinning cell for electrowinning a metal from solution, the cell having a spaced inlet and outlet and a substantially cylindrical cathode surrounding an anode defining a flow passage therebetween, the method including:
passing a metal containing process solution through the flow passage from the inlet to the outlet while a voltage is applied across the cathode and anode so as to deposit particulate metal from the solution on the cathode;
periodically interrupting the flow of solution through the cell and passing a flush solution in a reverse direction through the cell, the flush solution dislodging metal powder from the cathode and washing it out of the cell and into a metals recovery section of the plant.
The method may include the further step of recovering the particulate metal from the flush solution, eg in a metal recovery section of the plant.
Advantageously, the method further includes the step of interrupting the flow of flush solution when the particulate or powder metal has been removed from the cells and restoring the normal flow of solution through the cell to plate out further copper.
The method may include flushing the cells after 1 to 6 hours of pumping process solution through the cells, typically 2xc2xd to 4xc2xd hours of passing process solution through the cells. Typically, the flush solution is passed through the cell for 15 to 30 seconds, preferably 20 to 25 seconds.
Preferably, the process solution is passed through the cell at flow rate of 1,000 to 3,500 liters per hour (624-925 gallons per hour), preferably 2,000 to 3,000 liters per hour (5.28-792 gallons per hour), and the flush solution is pumped through the cell at a flow rate of 6,000 to 10,000 liters per hour (1585-2642 gallons per hour), preferably 7,000 to 9,000 liters per hour (1849-2378 gallons per hour).
Typically, the flush solution is pumped through the cell at a higher pressure than the process solution. This higher pressure assists in dislodging metal powder from the cathode.
In a typical cell during normal operation the metal containing process solution travels up the cell from the inlet to the outlet and the flush solution travels in a reverse direction down the cell from outlet to inlet. This way gravity assists in dislodging the powder metal from the cathode and in washing it out of the cell.
The method may also include periodically passing a mechanical cleaner through the flow passage to remove any plate or other solid dendrites or the like which may have plated out on the cathode.
The method may also include passing bubbles, eg air bubbles, up through the flow passage of the cell, eg after the flow of process solution has been interrupted and before the flow of flush solution has been started, to assist in dislodging powder metal from the cathode.
According to yet another aspect of this invention, there is provided an electrowinning plant comprising a plurality of banks of cells as described above with reference to the second aspect of the invention, the banks being operatively connected together such that process solution containing metal to be electrowon can be passed through each of the banks in series.
Typically, flush solution is passed in a reverse direction through the banks of cells.
Typically, the flush solution is only passed through a single bank of cells at any one time. It is not passed through all the banks in series in a reverse direction.
The plant may comprise at least three banks of cells in series. The exact number of banks for any particular application will depend on the initial grade of process solution and the target grade of the product solution as well as the current density in the cells.
Typically, only one bank of cells has the flow of process solution therethrough interrupted for flushing at any one time. That way the flow of process solution through the plant can be continuous, only one bank of cells being taken out of production for flushing at any one time.